Kudzu fails to find the device file for USB ACM modems, and as a result,
system-config-network doesn't find these modems either. I believe the fix is as
simple as adding
__getSysfsDevice((struct device *)t,".","tty:",0);
right next to
__getSysfsDevice((struct device *)t,".","net:",0);
in getUsbDevice() in usb.c

You wouldn't happen to have any USB-serial converters (ttyUSBX) and notice if
this works for them? :)
Moving the call to after __getNetworkAddr - otherwise if someone does something
patently silly (names a network device ttyACMX), we'll get bogus data.
Will be in 1.2.62-1, might be in a future FC6 update.